1. Field of the Invention
This invention relates to a light emitting device that a light emitting element is mounted on a base material and sealed with rectangular solid glass. Also, this invention relates to a light emitter using the light emitting device.
2. Description of the Related Art
Conventionally, a light emitting device is known in which a light emitting element such as an LED (light emitting diode) is sealed with a transparent resin material such as epoxy or silicone-based material. Such a light emitting device includes a practical light emitting device that a ultraviolet, violet or blue LED chip is used as the light emitting element, and a phosphor to be excited by light emitted from the light emitting element is mixed into the transparent resin material so as to obtain white light (See JP-A-2004-111882 and JP-A-2005-093896).
JP-A-2004-111882 discloses a light emitting device that an LED chip is mounted at the bottom of a concave reflection mirror, a low-concentration resin layer including a phosphor at low concentration is formed up to the top surface of the LED chip and a high-concentration resin layer including a phosphor at high concentration is formed on the low-concentration resin layer. The form and the phosphor concentration of the low-concentration resin layer and the high-concentration resin layer are adjusted such that a value is substantially constant which is given by multiplying by the concentration of the phosphor the optical path length of light emitted from the LED chip until reaching the interface of the high-concentration resin layer and an external resin material.
JP-A-2005-093896 discloses a light emitting device that an LED chip is mounted at the bottom of a concave portion formed in a base material, a first wavelength conversion material with a phosphor dispersed therein is filled halfway in the concave portion, and a second wavelength conversion material with a higher phosphor concentration than the first wavelength conversion material is filled on the first wavelength conversion material.
However, the light emitting devices of JP-A-2004-111882 and JP-A-2005-093896 have a problem that the transparent resin material is likely to deteriorate by light or heat discharged from the light emitting element. Especially, when using the light emitting element of group III nitride-based compound semiconductor to emit a short wavelength light, the transparent resin material near the element may be yellowed by high-energy light and heat from the light emitting element, so that the light extraction efficiency deteriorates with time.
WO-2004/082036 A1 discloses a light emitting device that glass is used as its sealing material so as to prevent the deterioration of the sealing material. In this light emitting device, a plate glass material is bonded by hot pressing onto a ceramic substrate on which an LED chip is mounted so as to seal the LED chip. After the hot pressing, the glass with the substrate is separated or cut by using a dicer etc. so that a glass sealing part is shaped like a rectangular solid on the substrate.
In the light emitting device of WO-2004/082036 A1, since the glass sealing part is formed by cutting the glass material by the dicer during the production, it is preferable to use the thinner glass material so as to prolong the exchange time of the blade of the dicer. Further, in case of using the glass sealing part including a phosphor, since the glass material with phosphor dispersed therein is expensive, it is preferable to use the thinner glass material so as to reduce the manufacturing cost.
However, in case of the thinner glass sealing part with the phosphor dispersed therein, unevenness in emission color will be caused. This is because the distance from the mounting location of the LED chip to the side surface of the glass sealing part is shorter than that to the upper surface of the glass sealing part, so that chromaticity difference is produced between light discharged from the upper surface of the glass sealing part and light discharged from the side surface of the glass sealing part. For example, if light discharged from the upper surface of the glass sealing part is set to be white light, light discharged from the side surface thereof will be yellowish. If light discharged from the side surface of the glass sealing part is set to be white light, light discharged from the upper surface thereof will be bluish.